Michigan Minds podcast: Dealing with plastic that can’t be recycled

April 4, 2024




APRIL 4, 2024


Morgan Sherburne:

Welcome to the Michigan Minds Podcast, where we explore the wealth of knowledge from faculty experts at the University of Michigan. I’m Morgan Sherburne, a public relations representative for the Michigan News Office. I want to welcome Anne McNeil, a professor of chemistry in macromolecular science and engineering to the podcast. Today we’re going to talk about microplastics, plastics, and the future of recycling. Hi, Anne. Thanks so much for being here with us today.

Anne McNeil:

Thanks for having me.

Morgan Sherburne:

Can you talk a little bit about the difference between the kind of recycling we do now and the recycling that you work on in your lab?

Anne McNeil:

The form of recycling that most people are familiar with is the recycling that happens with the bins that are in the hallway. Most people put in different plastic products and then that gets shipped off to a facility where they sort them and then they break them down into smaller pieces and then melt them into fibers and pellets. And then, those ultimately become new products. And the problems with this form of recycling, which is called mechanical recycling, is that it’s very limited to essentially three different types of polymers that we use, polyethylene, polypropylene, and polyethylene terephthalate, which is the polymer that’s used in water bottles and other disposable drink beverages.

And the problem with mechanical recycling, even though it is great that we’re reusing it again, is that the quality of the materials degrades during the process of the mechanical recycling. And as a result, we can’t make a new water bottle from an old water bottle. Instead, we’re resorted to making things like park benches and other products that are eventually going to end up in the landfill after the next use. And that’s really one of the things that we wanted to address in our form of recycling, which is known as chemical recycling.

And it sounds just like the name, we use chemistry to do recycling of that polymer. And we tend to focus on polymers that can’t undergo mechanical recycling. So, ones that were just throwing in the landfill already and we do a chemical transformation on that polymer to try to make a new product that has either the same value as the original polymer that’s going in, or in the best case, it’s got more value. So, we’re creating higher value products from waste materials.

Morgan Sherburne:

Can you give a couple examples of the specific products that are just thrown into the landfill that you’re now finding ways to recycle?

Anne McNeil:

Yeah. Two of the projects we’ve worked on in our lab were both motivated by this question. Right now, we’re just throwing all of this plastic into the landfill because it can’t be recycled for various reasons. And one of those products is the super absorbent material that’s used in diapers and absorbent high chain products that can’t be recycled currently because it has these things called cross-links. It’s like a fishnet. Instead of having long, linear polymer chains, which is most of our polymer products, the diapers have this fishnet quality where all those long chains are linked together. And that makes it basically impossible to do the mechanical recycling because you can’t melt those materials.

And then, another polymer we’re interested in in my lab is called polyvinyl chloride or PVC, familiar to probably a lot of people because it’s used in piping and a lot of applications, flooring. And that can’t be recycled mechanically because it often will off-gas hydrochloric acid during the recycling process when you heat it. And that can be a safety hazard for both workers and the materials in the production facility. And also, PVC products are filled with a bunch of additives and materials called plasticizers that make them softer. And we don’t want any of those additives or plasticizers in the waste stream because they, again, will degrade the quality of all the other plastics that are mixed in there. So, many plastics on the market right now, and actually in your life, can’t be recycled and they just get sent in the landfill. And those are the ones we want to go after with chemical recycling.

Morgan Sherburne:

You’ve had an interesting project with something a lot of parents of young children struggle with, diapers. Can you take me through how you’re repurposing diapers?

Anne McNeil:

Diapers are multi-component materials, meaning that they have multiple polymers put together into a single product. And so, the very first stage of recycling a diaper after you’ve done the sanitation on it and made sure that it’s healthy and safe to work with the materials, you have to take out the outer layers. That’s the comfort layer, it’s the fabric that is next to the child’s skin and the supporting layer on the outside to keep all the materials together. That’s typically a polyethylene or polypropylene material, and that can be recycled through mechanical recycling.

On the inside of the diaper, there are two materials. There’s something that quickly absorbs water and then something that holds onto the water over a long period of time, but more slowly. The quickly absorbing material is often cotton, and that also can be biodegraded or composted. But the more strongly absorbing material, the thing that holds onto the water so when the kid sits down, it doesn’t just squish all out, sorry for the image there, but the absorbing polymer is this cross-linked material where it’s like the polymer chains… Most polymers and polymer products are these really long, linear chains like a thin of spaghetti.

But in diapers, there’s these cross-links between those long chains and it’s kind of like a fishnet, but many fewer cross-links, so maybe a really big, gappy fishnet. And those cross-links are what prevent you from doing a lot with it, so you can’t dissolve it. And so, it makes it really hard to do chemistry on it. So, the very first thing we did with the super-absorbing material was cut those cross-link bonds. And we did very simple chemistry that could be done on this table here with stuff you can buy from the grocery store, just use a little bit of acid and some water and heat it up, and that breaks down the cross-links.

So, what we’re left with now is a really long chain that we can now turn into a lot of different products. And in diapers, the polymer that’s used to absorb the water has already what we call a functional group on the side of the long, linear chain. And that functional group is an acid group. That alone is kind of useful, but we thought if we had converted that in one step to a different group, it’s called an ester. It’s not super important, but just changing that functional group and doing one chemical step on it made it into a polymer that’s widely used and sold all over.

And it’s known as what’s called a pressure-sensitive adhesive, but it’s probably most familiar to people as the sticky adhesive material on the back of labels, and Post-It notes have this type of pressure-sensitive adhesive where if you put a little bit of pressure from your finger, it will stick. And so, we were able to convert the diaper super absorbent material into this sticky adhesive material, which already has an existing market, just through these two chemical steps. So, one was cutting those cross-links, and then one was changing the nature of that functional group on the backbone.

Morgan Sherburne:

Your lab made a serendipitous discovery with that adhesive that you’ve been making from recycled diapers. Can you tell that story?

Anne McNeil:

Yes. Such a wonderful story. The graduate student who was doing the recycling, I should mention his name, Takuna [foreign language 00:06:37], he was working with a high school student, Edwin Zishiri. And Edwin was working on coming up with ways to chemically recycle vulcanized rubber, so tire particles. And the vulcanized rubber that we got from a donation from a tire recycler was micronized. So, it was already cut up into tiny little pieces, otherwise known as microplastics. It was really small pieces of rubber.

And so, he was using that in his reaction that he was working on. And Takuna was working on his diapers. But they shared a waste bottle in the hood. And so, at the end of each reaction, when they were done analyzing all their materials and stuff, they would throw the stuff into a waste bottle. And so, at the end of the night, Edwin had thrown his vulcanized rubber materials into the waste bottle and the bottle was all black. And Takuna had also thrown some of his adhesive in there from earlier in the day, and they left.

And then, the next morning, Takuna came in and the bottle was completely clear and transparent, and there was a little floating piece of what looked like gum, but it was the adhesive and it was coated in black particles. And so, basically, during the night, without any stirring or anything, the adhesive blob attracted all of the microplastic particles of the rubber onto it. And right away, Takuna sent me the picture over Slack. And right away, I was just like, “This is awesome.” And I called Takuna and we talked about it, and we were both super excited. And we actually started… The next chapter of his thesis was on developing capture devices for microplastics in water using the adhesive that he made from diapers.

Morgan Sherburne:

Where does this research project stand now?

Anne McNeil:

When Takuna was in the lab, he was coating beads, these small beads, and the idea was that we were going to fill a column with these adhesive coated beads and then use that as a water filtration device. But for various reasons, the bead substrates were not… They were good, we could remove 99% of microplastics in like 10 minutes, but they weren’t great for coating. They just didn’t have the uniform coatings on them. So, I had a postdoc come in afterwards, [foreign language 00:08:30], and she switched from adhesive coated beads to stainless steel mesh. And she literally bought off of Amazon just one of those scrub brushes and unrolled it, and then just spray coated it with an adhesive.

And she developed a whole paper, that actually just came online last week, where she coats the stainless steel mesh with adhesive and then shoves that into a little flask, and then we pass water through that. And we were able to show it in this paper, along with the help of three other graduate students because [foreign language 00:08:56] had left before we finished this paper. I want to mention the other students’ names, Malavika Ramkumar, Henry Thurber and Maddie Klo were all working on this and helping take this project to the finish line.

But we were able to show that we could go as low as the concentration of microplastics is found in drinking water today, and use the same small sizes that are found in drinking water, and we could capture almost all of them. It was like 90% of the microplastics from authentic, but simulated drinking water were able to be captured with this filter. So, that was really cool. And Henry is taking on this project further in many directions, looking at different adhesives, different mesh sizes, and he’s trying to integrate that into a filter that you could attach to a water system at home.

This has really opened up a huge new area for my group. So now, we’re interested in microplastics not just in drinking water, but Malavika is looking at microplastics that are in our biosolids from the Ann Arbor Wastewater Treatment Plant because not a lot of people realize that the wastewater treatment plant is where a lot of microplastics are captured coming out of our households, like from laundry. And so, she’s looking at how much is in actually the Ann Arbor biosolids, and then where those biosolids are going. So, we’re working with some local farms in Michigan to watch and trace how microplastics go from the biosolids to the land application of those biosolids as fertilizer into the soil. And that’s one of her directions for her project.

And then, I also got so interested in microplastics that I started collaborating with some other faculty on campus, and we have a grant from LSA called Meet the Moment where we’re measuring and modeling and mapping across the whole state of Michigan microplastics pollution in our air. So, this little adhesive discovery of microplastics has really ballooned into multiple projects in different areas to microplastics in air, microplastics and soil, and microplastics in water.

Morgan Sherburne:

There always seems to be a question about whether replacements for single use products such as metal straws or usable grocery bags. Some studies have found that they require more energy to produce than simply using disposable products. Can you talk about the cradle to grave life cycle of disposable products?

Anne McNeil:

This is an extremely hard question for people even like myself to answer. When I understand all of the technical challenges and even coming up with an answer, it’s such a hard thing to get enough information on to feel good about the answer. You can Google, for example, metal straws versus plastic straws, and the very first thing that will come up is the emissions needed to make those two different products. And that’s what everybody focuses on first, is how much carbon, greenhouse gases, carbon emissions were produced during the production of a single metal straw to a plastic straw. And then, how many times would you have to use that metal straw then in order to make up for that. Because most often, the things that are more durable, like the metal straws, require more energy to make than a plastic straw. This is why plastic is the choice of so many products.

And so, for metal straws, I looked it up last night, it was like 150 times you would have to use it to match the emissions for one plastic straw. So, that’s one way of looking at it, and that’s probably the simplest way of looking at it. But if you dig just a tiny bit deeper and you think about, okay, well, where was the source of that metal? Did we have to go mine nickel out of some mine somewhere? And are we accounting for the safety of the workers and the emissions that were in the mining and acquisition of those starting materials, versus, where did the oil come from? And where was that processed? And what was involved in the acquisition of the oil and then doing the processing?

And then, there’s the disposal factor, which is what you’re talking about with the grave. A lot of the times people focus on just the production process, and then you could ask questions about the beginning and where the materials came from. But you can also ask questions about the end, and where is this metal straw going to go or where’s the plastic straw going to go when we’re done with it? And that’s the grave part, actually taking into consideration the whole life cycle of the product, not just the use time or the production type. And so, for metal straws, actually they do have an advantage here because they could be recycled, there is recycling for stainless steel already in this country, and there’s a process there.

The plastic straw is not going to get recycled, those are going to go into a landfill. And so then, you have to also think, how often are you going to be recycling the straw? If you hold onto it for 10 years and use it all the time in place, then that’s probably, in the end, going to be better than the plastic straw. But if you’re only going to use the plastic straw twice during the year, then you don’t need a metal straw to replace that. So, it’s an issue that is hard to wrap your brain around because there’s so many complex factors. And then, it also just depends on how frequently you’re going to use it.

I noticed that there was a whole discussion online about the irony of people buying things so that they can be more sustainable, because there’s more consumption involved in buying that metal straw in the first place. And if you’re not a big straw user, then don’t buy the straw, right? So, it’s a tough question, and I think every single one of these questions, when it comes up, you have to get as much information as you can about the entire life cycle, from getting the started materials, to the production, to where it’s going to go when you’re done. And you also just have to think about your own usage of it and whether or not it’s truly acting as a replacement for something and you’re going to actually commit to using it a bunch and you’ll commit to recycling it when you’re done.

Whenever people ask me this question, I always say, think about just reducing the consumption in the first place. So, if you can avoid straws altogether, I know straws are essential for some people, but if you can avoid straws altogether, that’s the easier solution than trying to find a replacement for it. It’s better just to reduce the consumption in the first place.

Morgan Sherburne:

Can you talk a little bit about microplastics in our drinking water?

Anne McNeil:

Most people probably don’t realize that they are drinking microplastics right now in our drinking water, but there’s been a lot of studies across actually the whole globe, and it’s found in almost every single tap water sample, no matter where you are on the planet. And the microplastics are getting there from the environment, there’s microplastics that we release into the water through our laundry. There’s microplastics that are generated out in the environment from the breakdown of larger plastic products. We have microplastics in our air that are depositing on the drinking water even after it’s been treated when it’s sitting out in those big treatment bins.

And so, it’s really hard to avoid encountering microplastics in our lives. It’s in our food, in our air, in our water. But one thing that we can, as individuals, push for is more regulation of microplastics in these various environments. So, there should be microplastics legislation for laundry machines. That’s one of our biggest effluents out into the environment. And there should be a mandate. We have lint traps for our dryers, but we don’t have one for the washing machine. They can just wash all those microfibers out of your clothing out into the stormwater or wastewater treatment facilities. And that just goes out into the environment as well, as I was talking about earlier. And so, we should push for more legislation minimizing sources of microplastics.

At the same time, I think we should also push for some regulation and legislation monitoring our drinking water. Right now, California is the only state that has set a limit for microplastics in drinking water, and they can’t even enforce it yet because they’re still working on the technologies to meet those limits. But they have a law that says it has to be below a certain limit and we need more states to move in that direction. So, I think one of the things as an individual you can do is try to push for your state senators and legislature, and then even the federal government, to start thinking about regulations and implementing them so that we can minimize the amount of microplastics we’re intentionally dispersing into the environment and minimize the amount that we’re bringing into our bodies.

Morgan Sherburne:

Why are microplastics harmful for our bodies?

Anne McNeil:

This is a great question too, and I had a student last weekend during recruiting ask me this question as well. It’s hard to do studies where we find out the exact harm that microplastics are causing humans because you don’t want to intentionally dose humans with microplastics. However, there’s been a number of studies where people are looking at various parts of the body and they find microplastics. There’s a study where they were looking at lung tissue at different depths of the lung and they found microplastics in every depth of the lung. There was just a study that came out last week of they were imaging plaques that they had taken out of arteries from different patients, and the patients who had suffered a cardiac event had more microplastics in their plaques.

There’s been other studies that have shown microplastics in blood in our stools. Microplastics are already entering our body. And it’s still a little bit unclear, it’s hard to say, “Okay. For sure this microplastic concentration in the lungs is leading to this disease.” Or anything like that. But there’s plenty of evidence in the animal world, where they have done more controlled studies. And especially in the marine environment, there’s been a lot of studies on fish species and what microplastics have done to their developmental abilities and even survival.

And so, there’s plenty of data out there that says that having them in our bodies is not probably a good thing, whether you’re looking at animal studies or even some of the studies now that they’re already in us. So, I find it personally very concerning. Even if there isn’t a direct tie, like this amount of microplastics will cause this disease.

Morgan Sherburne:

I’ve heard the term chemical recycling before. Does it all look like the type of chemical recycling that you were doing?

Anne McNeil:

Thank you for asking that because I should have clarified this earlier. People use the term chemical recycling to refer to a lot of different processes. Just to give you a couple examples of it, people can burn the plastics and recover energy from that, and that’s called incineration or waste to energy recovery. Some people try to label that as chemical recycling. And then, other processes where people break down plastics using, again, high intense energies and catalysts to make things that are fuels and then they burn those fuels from the plastics. That’s also sometimes called chemical recycling.

So, I think that is a catch-all term for everything that’s not being recycled through the mechanical, where we just simply melt things and try to make new polymer products. I think most people use it when the chemical nature of the polymer is being changed in any fashion. And the part that concerns me a little bit, and I always try, when I talk about this, to focus on my form of chemical recycling, which is try to keep the value in the material. We’re not trying to make fuels or waxes or things that we’re going to burn.

I try to point out the difference between our chemical recycling, which is a really intentional, single, chemical transformation happening on the polymer, and we still have a polymer at the end and we can make a plastic product out of it. Versus these other forms where it’s really destruction and often destruction followed by burning or using it as a fuel. I try to separate those two forms of recycling, although the term is being widely used. I just encourage people to pay attention when they hear chemical recycling, that’s not always the neat, clean chemistry that we’re doing in the lab. It can sometimes just mean burning.

Morgan Sherburne:

Can you define what microplastics are and how they get into our environment and then into our bodies?

Anne McNeil:

Microplastics are defined based on their size, and it’s a range of sizes. So, the smallest size microplastic is what’s called one micron, and that’s about 1/70 the width of your hair, because your hair is about 70 microns. And then, the largest size range for microplastics is 5,000 microns, which is about the size of a pencil eraser. And so, anything that fits in that size range is considered a microplastic. If you’re smaller than that, that’s what’s called nanoplastics. And if you’re larger than that, then that’s just kind of… We call it a macroplastic just to distinguish from microplastic, but it’s a large plastic fragment.

And the way microplastics are generated, there’s really two sources. There’s what’s called primary microplastics, and those are plastics that were intentionally manufactured to be that size. A long time ago, we used to make these microbeads and cosmetic products, and they were intentionally synthesized and made to be those tiny little beads. There’s not a lot of products that are still made on that size because, thankfully, it’s been banned from a lot of products, but we still do make some primary microplastics.

The second type of microplastics are what are called secondary microplastics. And so, these are plastics that were shed from larger plastic items. And it’s typically shed through abrasion or physical breakage of a plastic item. A couple of good examples of this would be like when you’re driving down the street and your tire is rubbing on the road, and maybe you break or you don’t break, it doesn’t really matter, just that rubbing action of the tire on the road generates microplastics. And that’s one of the biggest sources of microplastics out in the environment, is actually micronized rubber coming off tires just from driving.

And it’s kind of easy, I think, for a lot of people to realize that that’s happening because you have to replace your tires every couple of years. So, all of that tread went somewhere, and it’s out on the edges of our streets and our streams and our grass, in the air, actually, as well. Another example is actually when you wash your clothes in the laundry machine, all that physical abrasion of the clothing, little fibers break off of your clothing over time, which is why your fleece sweater feels really soft. And then, you wash it and it’s not as soft anymore. It lost a bunch of those fibers during the process of laundry. And then, that ultimately ends up also out the environment.

And then, any other plastic products that are out there, everything breaks down. Plastic doesn’t last forever. And so, plastic floating in the ocean and on our lakes, those can break down hitting rocks or waves or just each other, other items. So, it’s just the fragmentation of larger plastic items.

Morgan Sherburne:

How does it work its way into our bodies?

Anne McNeil:

Great question. We are exposed through microplastics through three main routes. The first is breathing. There’s microplastics in our air. Indoor air tends to be more contaminated than outdoor air, so if you spend more time outside, that’s good for you in terms of microplastics inhalation. But inhalation is one form of our exposure. The next form is through drinking. Water, I’ve already mentioned, has microplastics in it, but so does your beer and your wine and any beverage or soda. All of those things have microplastics in it. And those microplastics either came from the water it was sourced from or during the production facility.

The study that we read about beer, it was actually likely the fabric from the clothing of the workers during the brewing process that ended up being the largest contaminant inside the beer. So, you’re drinking a bunch of microplastics. Whatever you’re drinking, it’s got microplastics in it. And then, the final source is through ingestion. So, the food you’re eating can have microplastics in it. Either the food was grown on soil that has microplastics, or it’s grown out in the environment in microplastics from the air deposited on it.

And fish. For example, lots of studies in fish now of microplastics finding their way into their muscles from their ingestion. So, they eat the microplastics that are in the water, and then through their digestion process, it ends up filtering out into their liver and into their fillets. Then we eat the fillet, and then we eat the microplastics. So, your three main exposure routes, eating, drinking, and breathing. And these are all essential for life, so you can’t really avoid them so much once they’re there.

There’s definitely studies I’ve looked at. Bottled water is often worse than tap water because you’re bottling it in a plastic item, and often people have also seen that the screw cap, just the action of screwing and unscrewing bottled water leads to microplastics generation. So, you can choose tap water over bottled water. There’s small choices you can make. But you can’t ever keep microplastics out of your body, unfortunately. They’re just with us. Sorry. I’m like the most depressing person to talk to you.

Morgan Sherburne:

No, no, no. [inaudible 00:24:19]. Anne, thanks so much for being here today.

Anne McNeil:

Thank you for having me.

Morgan Sherburne:

Thank you for listening to this episode of Michigan Minds, produced by Michigan News, a division of the university’s office of the vice president for communications.

Consumers have to wrangle with a sticky issue: Much of the plastic used every day can’t be recycled.

And the kind of recycling that can be done is called mechanical recycling, which means that plastic that can be recycled is simply broken down to be repurposed as other plastic objects, often which are of a lower value than the original product. Eventually, objects made out of this recycled plastic, such as park benches, just end up in a landfill.

But University of Michigan chemist Anne McNeil is focusing on how to recycle previously unrecyclable plastic, using chemistry to modify the plastic into a product of equally high value to the original product.

In this new episode of Michigan Minds, McNeil discusses how her lab approaches chemical recycling and the microplastics rabbit hole her work has led her down, including looking at microplastic pollution on local farms and airborne microplastic pollution across the state of Michigan.

Can you talk a little bit about the difference between the kind of recycling we do now and the recycling that you work on in your lab?

The form of recycling that most people are familiar with is the recycling that happens with the bins that are in the hallway. Most people put in different plastic products and then that gets shipped off to a facility where they sort them and then they break them down into smaller pieces, and then melt them into fibers and pellets. And then those ultimately become new products.

The problem with this form of recycling, which is called mechanical recycling, is that it is very limited to essentially three different types of polymers that we use, polyethylene, polypropylene and polyethylene terephthalate, which is the polymer that’s used in water bottles and other disposable drink beverages.

The problem with mechanical recycling, even though it is great that we’re using it again, is that the quality of the materials degrades during the process of the mechanical recycling. And as a result, we can’t make a new water bottle from an old water bottle. Instead, we’re resorted to making things like park benches and other products that are eventually going to end up in a landfill after the next use.

And that’s really what we wanted to address in our form of recycling which is known as chemical recycling. And it sounds just like the name. We use chemistry to do recycling of that polymer and we tend to focus on polymers that can’t undergo mechanical recycling.

Contact: ajmcneil@umich.edu